1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a sheet of paper and an image forming system including the same.
2. Description of Related Art
An image forming apparatus equipped with multiple functions, which is simultaneously provided with various functions such as a printer, a scanner, a copy machine and a facsimile, has been widely used in recent years. In such an image forming apparatus, it is very important that a position of a sheet of paper transported from any of feeding trays or the like is accurately aligned so that an image can be suitably transferred in an image forming unit. Thus, for example, the image forming apparatus has registration rollers at an upstream side of a secondary transfer portion which transfers the image on the sheet of paper. The registration rollers then correct any oblique movements of the sheet of paper by hitting a forward end of the sheet of paper against the registration rollers or any deviations of the sheet of paper by shifting the sheet of paper to a reference position, so that the image forming apparatus controls a transported position of the sheet of paper with high accuracy.
FIGS. 1A through 1C respectively show a correction of the deviations of the sheet of paper, which has been performed in the past. As shown in FIG. 1A, a deviation detection sensor 300 detects a position through which a transported sheet of paper P passes. A controller, not shown, calculates an amount of deviation ΔLa of the sheet of paper P on a direction D2 which is orthogonal to a transportation direction D1 of the sheet of paper P, as shown in FIG. 1B, based on the position of the sheet of paper P detected by the deviation detection sensor 300 and a reference position SD which has been previously set. A registration and fluctuation unit 360, which has a pair of rollers and is provided at an upstream side of the secondary transfer portion which transfers the image on the sheet of paper, stops the transportation of the sheet of paper P and holds the sheet of paper P between the pair of rollers to shift it back to the reference position SD along the direction D2 by the amount of deviation ΔLa which is calculated by the controller as shown in FIG. 1C. Accordingly, the sheet of paper P is shifted back to the reference position SD, thereby enabling the deviation of the sheet of paper P to be corrected.
When controlling the transported position of the sheets of paper P with high accuracy, however, a lot of the sheets of paper which have the same size are fed passing through the same reference position in a fixing unit provided at a downstream side of the secondary transfer portion. Any scratches may occur on fixing rollers constituting the fixing unit by the edges of sheets of paper P. These scratches cause any stripped strains or blur to occur on the image, so that a poor printing sheet(s) of paper may occur. Thus, life of the fixing unit having the fixing rollers becomes short and an exchange span of the fixing unit also becomes short.
In order to prevent the scratches from occurring on the fixing rollers, a technology in which the sheets of paper are intentionally shifted to change their passed positions has been known. For example, Japanese Patent Application Publication No. 2010-2653 has disclosed an image forming apparatus in which a shift unit drives a pair of registration rollers to shift a reference position of detecting means for a sheet of paper transported to a fixing unit by some distance for every group of a set number of transported sheets of paper so that any abrasion by the edges of the sheets of paper in the fixing rollers is decreased.
However, the past image forming apparatus, which has been disclosed in Japanese Patent Application Publication No. 2010-2653, has a configuration such that when performing in-line processing on sheets of paper ejected from an image forming apparatus in a post-processing device, the image forming apparatus stacks the transported sheets of paper under its alignment function and put one sheet of paper on top of another sheet of paper with high accuracy. Further, in this image forming apparatus, since the sheets of paper are transported to the post-processing device with their reference positions being shifted on the basis of the correction of the deviation of each of the sheets of paper in the image forming apparatus, flexible thin sheets of paper are, particularly, aligned with less accuracy.
FIGS. 2A and 2B show an example of paper alignment processing of the sheets of paper in the past post-processing device. The sheets of paper P which have been shifted by the shift unit of the image forming apparatus are transported into an alignment portion composed of a pair of alignment plates 400, 400 in the post-processing device. The alignment portion aligns the sheets of paper P by moving the alignment plates 400, 400 from their home positions to their alignment positions. In this alignment processing, as shown in FIG. 2B, when the sheet of paper is small sized one or a thick sheet of paper (they are referred to as “sheet of paper P1”), the sheet of paper P1 has large stiffness (is stiff) so that if the pair of alignment plates 400, 400 align the sheets of paper P1, the pair of alignment plates 400, 400 can align the sheet of paper P1 correctly. On the other hand, when the sheet of paper is large sized one or a thin sheet of paper (they are referred to as “sheet of paper P2”), the sheet of paper P2 has small stiffness (is flexible) so that if the pair of alignment plates 400, 400 align the sheets of paper P2, each of the sheets of paper P2 is folded or flexed, which causes its alignment accuracy to be considerably decreased. This leads to a deterioration of final finishing accuracy.